The invention relates to a control of the ultra-filtration in connection with hemodialysis. A control apparatus controls the alternate supply or withdrawal of fresh and used dialysis solution into one or the other or out of one or the other chamber of at least one balancing apparatus and for controlling the drive of an ultra-filtration pump for the additional withdrawal of used dialysis solution out of the dialyzer circulatory system.
A known method for the discontinuous measuring of the liquid withdrawal per unit of time resides in that the dialyzer solution flow to the dialyzer is temporarily interrupted by means of switch-over valves and in that the outflow conduit of the dialyzer is connected with a through flow meter of reading the liquid withdrawal per unit of time. By means of additional steps the pressure difference between the blood side and the dialyzing solution side is kept constant on the previous operational value. Thus, the passage of liquid through the membrane during the measuring is about the same as during the normal operation. It is disadvantageous, that each time it is necessary to interrupt the operation so that a continuous measuring is not possible according to said method. Further, with said method only one measuring possibility is provided. In order to achieve a determined value of the liquid quantity to be withdrawn it is necessary to evaluate the measured results and to make a corresponding manual correction of the operating values in the apparatus at relatively short time intervals.
It is known from U.S. Pat. No. 4,113,614 to automate the control of the pressure across the membrane in a hemodialysis apparatus and also to automate the control of the ultra-filtration rate by metering and monitoring the ultra-filtration rate and the total quantity of ultra-filtrate removed. One embodiment of this U.S. Pat. No. 4,113,614 which was issued on Sept. 12, 1978, requires a dialysis solution reservoir having a constant volume. In another embodiment of this prior art two pumps are used having exactly corresponding pumping rates for the supply and discharge of dialysis solution, whereby a dynamically closed system is provided. However, it is in practice very difficult to construct pumps in an economically feasible way, which have exactly corresponding pumping rates.
U.S. Pat. No. 4,113,614 also discloses a special pump for the withdrawal of liquid from the dynamically closed system in such a quantity that the quantity of withdrawn liquid corresponding to the ultra-filtration quantity. A piston pump is disclosed for this purpose which is also utilzed simultaneously for measuring the pumped quantity and, based on the assumed proportionality of the driving force for the pump and the working pressure of the pump, for measuring the pressure. This type of apparatus leaves room for improvement, especially with regard to the problem of proportioning the concentrate and water. More specifically, the preparation of the dialysis solution from concentrate and water in exactly predetermined ratios in combination with the supply and removal of liquid quantities corresponding to each other may be improved upon, particularly if these mentioned partial functions are to be directly combined or linked with each other as is the purpose of the present invention.
An article by Harold P. McDonald, Jr., entitled "Automatic Peritoneal Dialysis Machine For Hospital Or Home Peritoneal Dialysis; Preliminary Report" appeared in volume 15 of the "Transactions of the American Society of Artificial Internal Organs", 1969 starting at page 108 and ending on page 111. A peritoneal dialysis apparatus is not necessarily suitable for hemodialysis purposes. For example, in a peritoneal dialysis apparatus a single state system is used in a discontinuous operation with piston pumps. The ultra-filtration is not controlled and the ultra-filtrate quantity may be measured only at certain times, namely, at the end of the flow-out time by measuring the liquid level in the flow out container. Thus, the teachings applicable to a peritoneal dialysis apparatus are not suitable for a hemodialysis apparatus especially where the latter is supposed to operate continuously in a push-pull fashion to assure a continuous, uniform flow through the dialyzer. Moreover, the control of the ultra-filtration with a predetermined ultra-filtration rate in terms of ultra-filtration quantity per unit of time, as well as a continuous indication of the accumulated ultra-filtration quantity, is not taught by McDonald, Jr.